The present invention relates to a method for aligning a laser machining apparatus, more specifically to a method for aligning a laser repair apparatus for machining a wiring pattern on a semiconductor wafer.
Recently, as semiconductor device fabrication techniques have been developed, semiconductor devices have become more integrated and have gained higher densities, and circuit patterns have become more micronized. Accordingly, circuit defects tend to increasingly occur. To improve yields of IC chips with respect to such defect increases, a repair (defect remedy) technique of forming redundant cells in advance in an IC chip for replacing a cell which has become defective has become widely used. As one example of such a repair technique, there is known a method using a laser repair apparatus, which is a laser machining apparatus. A fusing pattern-portion (fuse) is prepared in a redundant cell, and a converged laser beam is applied to the fuse to break the pattern to replace the defective cell with the redundant cell.
The repair technique using the laser repair apparatus requires accurate alignment, and the apparatus must be structured so as to ensure high alignment accuracy and allow adjustment of alignment accuracy. Adjustment of alignment accuracy is performed usually by highly-skilled specialists, and aligning operations performed during machining are automated by the apparatus. When the adjustment of alignment accuracy is correct, the alignment can be automated with high accuracy.
However, alignment accuracy is often momentarily deflected when a machining operation is being performed, but repair apparatus operators then judge a small alignment deflection to be due to the intrinsic shape of the wafer because the alignment is usually automated, and they unknowingly continue the machining operation. Eventually, a disadvantage arises in that erroneous melting of fuses successively takes place.
A technique for preventing such successive erroneous melt due to alignment accuracy deflection is disclosed in Japanese Patent Laid-Open Publication No. 236989/1995. According to this invention, differences of measured positions of a plurality of alignment marks on the surface of a wafer-to-be-machined and a chip-to-be-machined and a reference position of an alignment mark preset in the apparatus are given; whether or not the differences are within an allowable value is judged; and when the differences exceed the allowable value, an alarm signal indicating that an alignment accuracy deflection has possibly occurred is outputted to thereby prevent the successive erroneous melt.
In the actual machining, even when a reference position of an alignment mark is displaced from a position of an alignment mark on a wafer-to-be-machined, such erroneous melting does not take place if coordinates of an alignment mark are correctly recognized, and a position of a fuse-to-be-machined is correctly recognized based on the coordinates. However, although the technique disclosed in Japanese Patent Laid-Open Publication No. 236989/1995 has the advantage of preventing successive erroneous melt due to deflection of alignment accuracy of a laser repair apparatus, it has the disadvantage any misalignment of a wafer itself might be judged to be deflection of alignment accuracy of the laser repair apparatus.